There are hundreds of thousands of tons of depleted uranium hexafluoride in temporary storage in the United States. A very small amount has been used in the manufacture of such things as tank armor, armor piercing artillery, shields for industrial radioactive materials and other applications.

         Research supported by the U.S. Department of Energy was begun in 1993 into the use of depleted uranium in heavy concrete. There was a dual goal in the research. They were looking for a way to dispose of large quantities of depleted uranium and also a new type of material for the creation of dry casks (covered in last post) used for storage and transportation of spent nuclear fuel. Scientists at Idaho National Engineering and Environmental Laboratory conceived of the new type of concrete and were awarded patents for it in 1998 and 200

          Ducrete is a special type of concrete that was invented to help solve the radioactive waste problem. The name is derived from depleted uranium concrete. Concrete is made from a gravel aggregate bound together with Portland cement, water and sand. In ducrete, the gravel aggregate is replaced by granular depleted uranium left over from the process of creating enriched uranium for nuclear reactor fuel or nuclear weapons. The depleted uranium is mostly U-238 with a much lower proportion of U-235 than natural uranium ore. Ducrete is a very efficient shielding material. The depleted uranium shields against gamma rays and the water and sand absorbs neutrons.

          There were a series of technical problems that had to be overcome in order to develop ducrete. Depleted uranium in the form of uranium hexafluoride is too reactive to be used in ducrete. It has to be oxidized to form depleted uranium trioxide (DUO3) or depleted triuranium octaoxide (DU3O8). Hydrogen gas is then used to produce uranium dioxide (DUO2) in a powdered form. The second issue is the fact that concrete requires aggregate or gravel. The powder that comes out of the chemical processing has to be turned into a granular form. The powder is heated but does not reach the melting point of the material. Individual atoms diffuse across the boundaries between the powder grains and form larger chunks called depleted uranium aggregate that are suitable for use in concrete.

          The ducrete can be used to build dry casks and containment storage building to store radioactive waste. Ducrete casks are smaller and lighter dry casks than containment buildings made of conventional concrete. Because of the additional shielding properties of ducrete, it is actually cheaper to produce ducrete dry casks than using steel, lead and depleted uranium metal for shielding in addition to concrete. While ducrete may be a good cheap alternative for temporary storage, the mild radioactivity of the ducrete casks makes them more expensive to dispose of than conventional dry casks.

          However superior ducrete may be for constructing dry casks and containment buildings, it has not actually been utilized for storage of spent nuclear fuel in the United States. No facilities utilizing ducrete have been licensed in the U.S.

ducrete.PNG

           When nuclear fuel has been burned in a reactor and there is no permanent disposal site, there is an alternative to storing the spent fuel rods after they have cooled off in a pool of water for at least a year. Spent fuel rods can be stored in steel cylinders in what is called ‘dry cask’ storage.

          Nuclear fuel rods are tubes of uranium oxide pellets. About two hundred of the tubes are packed together in an ‘assembly.” These assemblies are then inserted into the core of a reactor.

          When they are spent, they are removed to a spent fuel pool. After a year, the assemblies can be removed from the pools and sealed into steel cylinders called canisters that are around 16 feet long and 6 feet in diameter. The canisters are either welded or bolted shut.  Then the atmosphere in the canister is exhausted and replaced with an inert gas which is typically helium. Finally the steel canister is placed inside a ‘cask.’

          The cask containing a welded canister consists of a concrete cylinder with walls over two feet thick. The cask containing the bolted canister is a six inch shell of boron doped metal that acts as a neutron shield .

          The casks are stored in what are referred to as ‘Independent Spent Fuel Storage Installation’ which are licensed by the United States Nuclear Regulatory Commission. The first storage facility licensed by the NRC was the Surry Nuclear Power Plant in Virginia.

         The casks may be stood upright in concrete or steel buildings. Alternatively, they may be placed horizontally in chambers about the size of a one-car garage. As of the end of 2010, there were forty eight dry cask storage facilities licensed at reactor sites and fifteen dry cask storage facilities not located at reactor sites. About sixty two metric tons of spent nuclear fuel currently exist in the United States. Of that, about twenty two percent, or fourteen tons, is stored in dry casks.

         Dry casks can also be used to transport spent nuclear fuel to sites for permanent disposal when such a site is available. A typical design for truck transport is similar to the dry casks intended for storage but there are some differences. The diameter of the cask diameter is four feet which is expanded to six feet on the ends by disk shaped ‘impact limiters.” The overall length including the impact limiter disks is twenty feet. There is an inner steel canister surrounded by a lead gamma shield which in turn is enclosed in a second steel shell. The second steel shell is encased in a neutron shield and a final shell. Average weight is twenty five tons.

         Rail transport casks very similar in terms of shells and shields but are much bigger. Cask diameter is eight feet with overall diameter at eleven feet. The overall length is twenty five feet. The average weight of such an cask is about one hundred and twenty five  tons so it can transport five times as much as a truck cask.

         These casks should last at least a hundred years but they were only intended as temporary storage. The existing dry cask storage facilities may be targets for terrorists or vulnerable to natural disasters. Unfortunately with the cancellation of the Yucca Mountain Repository, the United States currently has no permanent spent fuel storage site.

Dry casks.PNG

          While the United States has been struggling with the Yucca Mountain site for nuclear waste disposal, other countries have been operating or planning repositories.

          Sweden has been operating a repository since 1988. Finland has one that has been in operation since 1992 and another that opened in 1998. Germany operated one that closed in 1995 and another that closed in 1998. The US has been operating a repository for transuranic wastes since 1999.

          There are repositories under construction in Finland, Germany and Korea. Repository projects are being discussed in Argentina, Belgium, Canada, China, Japan, and the United Kingdom. Germany has a proposed repository on hold, France and Switzerland are working on siting repositories and the United States has cancelled the only major repository for spent nuclear fuel that was being worked on.

          Radioactive waste was placed in the Asse II repository, a old salt mine in Germany between 1967 and 1978. The repository was officially closed in 1995. In 2008, it was reported that brine contaminated with cesium-137, plutonium and strontium had been leaking from the mine since 1988. This highlights one of the greatest worries about underground storage of nuclear wastes; contamination of the ground water around the repository.

          The other closed repository in Germany is the Morsleben repository in the Bartensleben rock salt mine that accepted waste from 1972 to 1998. The salt dome is collapsing and almost half a million cubic meters of concrete has been pumped into the pit to stabilize it temporarily. This highlights another great concern about underground repositories; geological stability of the geology around the repository.

          There was discussion of creating an international nuclear waste repository in Australia but the public backlash was so severe and long that the project was cancelled. The debate over the use of nuclear power and the disposal of nuclear waste arouses strong passions on both side of the issue. In Germany, where there is strong nuclear opposition, the recent disaster at Fukushima has generated such a public outcry that Germany has announced that it will phase out nuclear power generation completely. This is third great problem with geological repositories. While many people approve of the concept of underground spent nuclear fuel repositories in the abstract a lot fewer want one in their neighborhood.

           Of all the alternatives suggested for disposal of spent nuclear fuel, the deep geological repositories would seem to be the simplest and most easily manageable. The technical issues can certainly be addressed. Geological science can find places where the ground water will not be threatened and where the geology is stable. The biggest problem is not technical but political. The recent Fukushima disaster has caused a world-wide cooling of public enthusiasm. Given that the temporary storage for the world’s spent nuclear fuel may be completely full within five years, solution of both the technical and political problems around permanent disposal of spent nuclear waste must be answered as soon as possible.

Asse II salt mine in Germany:

Asse II salt mine in Germany1.jpg

          In 1957, the National Academy of Sciences of the United States recommended long term burial as the best solution for permanent disposal of nuclear wastes. Starting in 1978, the U.S. Department of Energy (DOE) has been studying Yucca Mountain in Nevada as a possible site for the first long term geological repository for U.S. The U.S. Congress passed the Nuclear Waste Policy Act in 1982 which gave the DOE the responsibility for locating a site, building an underground repository for nuclear waste  and operating the site on an ongoing basis. In 1984, the DOE selected ten sites in six states as possible locations for a repository.

           After the report on the sites was delivered in 1985, President Reagan selected three sites from the ten studied to go through an intensive review known as site characterization. Yucca Mountain was one of the three sites selected by Reagan. Congress amended the Nuclear Waste Policy Act in 1987 to focus exclusively on Yucca Mountain. The DOE contracted with utilities to begin accepting spent nuclear fuels at the Yucca Mountain Repository.

           In 2002, the United States Congress finally approved the Yucca Mountain Nuclear Waste Repository as a long term repository for spent nuclear fuel and other high-level radioactive wastes. In 2006, the DOE suggested if the project was fully funded that the Yucca Mountain repository could be opened and accepting nuclear waste by 2017.

          The plan called for forty miles of tunnels under Yucca Mountain which could be used to store a maximum of 77,000 metric tons of waste, include up to a maximum of 63,000 metric tons of spent nuclear fuel. Current estimates are that U.S. reactors will have produced that amount of spent fuel by 2014.

           Since the 1987 decision to drop two other proposed sites, the Yucca Mountain Project has been fiercely opposed by critics including two thirds of the people who live in Nevada. There have been protests, complains, laws suits, opposition by elected officials as the debate raged. The studies of possible ground water infiltration at the site were challenged and ultimately validated. Studies indicated that the cost of not moving forward with the repository would be very high.

          Democratic Nevada Senator Harry Reid became the Senate Majority leader in the 2006 mid-term elections when the Democratic Party took control of the Senate. He had been a long term opponent of locating a nuclear repository in his state and he used the resources of his new position to fight against it. The 2008 budget cut the Yucca Mountain repository budget to around four hundred million but the project managers did their best to rearrange their budget to continue work.

        In the 2008 presidential campaign, Barack Obama said that he would cancel the Yucca Mountain project. After he was elected, he was told by the Nuclear Regulatory Agency that the president did not have the authority to cancel the project. But in 2010, President Obama and Energy Secretary Chu did managed to cancel the Yucca Mountain Repository project.

         At this time, the U.S. has no long term storage for spent nuclear fuel. Spent fuel is being stored at reactor facilities in temporary water pools or dry cask storage. It is estimated that the temporary storage facilities at U.S. reactors will be full within 5 years. This is a critical problem facing the nuclear power industry in the U.S.

NRC Yucca Mountain Project diagram:

Yucca Mountain.PNG

1. Canisters of waste, sealed in special casks, are shipped to the site by truck or train.
2. Shipping casks are removed, and the inner tube with the waste is placed in a steel, multilayered storage container.
3. An automated system sends storage containers underground to the tunnels.
4. Containers are stored along the tunnels, on their side.

          A standard human way of dealing with something that you want to get rid of is to dig a hole and bury it. This has been a popular proposal of disposing of spent nuclear fuel and many countries that use nuclear power either have or are working on such repositories. The authorities in these countries claim that such repositories can be safe, economical and protect the environment but a large part of the public remains highly skeptical.

          Proposed repositories are general one thousand feet deep or deeper. The rock should be solid and dense. Stability is one of the most important factors. That means that there should not be any nearby volcanism. There should not be any geological faults in the area. A second very important factor is that there should be very limited movement of ground water through the rock strata to prevent threats to the water table. Finally, the repository should be as far from human habitation and land use as possible.

          Once the repository site has been selected and the tunnels dug, there will likely be additional preparation in the form of additional sealing of the walls of the tunnels. Salt mines have been a popular choice for conversion to nuclear fuel repositories because many tunnels and large areas have been excavated. Locations with a granite matrix are also popular choices.

            Concrete cylinders containing spent nuclear fuel rods would be transported to the repository site from the spent fuel pools, a process that has its own problems. The cylinders would be lowered into the repositories and arranged in a configuration that leaves room between the cylinders for heat dissipation. Eventually, when the repository is full, the access tunnel would be sealed. There have been discussions about how to leave a maker that would be comprehensible for thousands of years so that future societies would know that the site was dangerous.

          Belgium, Canada, Finland, France, Japan, Korea, Sweden, Switzerland, and the USA have created major underground laboratories to test the feasibility of deep geological repositories for spent nuclear fuel. Argentina, Belgium, Canada, China, Finland, France, Germany, Japan, Korea, Sweden, Switzerland, United Kingdom and the USA all have repositories filled and closed, currently operating, being planned or canceled.

         Whenever such a repository is proposed, after the serious effort to verify that all the technical details have been reviewed and all technical concerns have been satisfied, the hard work of selling the location to the public begins. Politics becomes the most important factor. On the positive side, there may be commercial interests who play down or even try to hide any potential problems. On the negative side, there may be environmental groups who over emphasize potential problems.

         Unfortunately, we do not have the luxury of kicking this can down the road. Current estimates are that the world’s spent nuclear fuel pools will all be full of spent fuel within 5 years at the current rate of nuclear fuel use. Deep repositories are the most realistic of the proposals for permanent disposal, despite technical and political problems. Time is running out to implement a disposal solution.

Planned repository in Finland:

Finland repository.PNG

          There are a number of different proposals for permanent disposal of spent nuclear fuel. Here are summaries of some possible techniques.

          Deep geological repository: This proposal consists of digging a deep hole in a geologically stable formation and burying sealed containers that would not leak into the water table. A depository in Yucca Mountain in Nevada was approved in 2002 and was under development until 2010 when it was defunded by an act of Congress.  

          Dry cask storage: In this technique, fuel rods are sealed into steel cylinders filled with an inert gas. The cylinders are welded or bolted shut and some are encased in a concrete, steel or a shell of another material to block radiation. These casks can be used for both transportation and storage. They can be stood on end or stacked horizontally in concrete and steel enclosures. This is considered to be only a temporary storage option pending development of a long term option.

          Ducrete: Ducrete is a special form of high density concrete. The name is derived from “Depleted Uranium Concrete”. Crushed ceramic pellets of depleted uranium dioxide is used in place of gravel in a Portland cement binder. The depleted uranium absorbs gamma rays and the water used in the cement absorbs neutrons. The Ducrete was developed as a means of shielding other nuclear waste and disposing of depleted fuel pellets.

          Ocean floor disposal: This alternative consists of burying spent nuclear fuel in the ocean floor where it won’t be disturbed by either geological or human activity. Some options discussed included dropping concrete encased waste into holes drilled in the ocean floor or using torpedoes to send the encased waste into the ocean floor. It would be difficult to recover the waste at a future time if that was desirable. There is an international treaty in force to prevent dumping radioactive materials in the oceans that will be in force until 2018 at which time such a disposal method could be considered.

          Deep borehole disposal: In this type of disposal, extremely deep holes would be bored into the earth as much as five miles. Some proposals call for lining the bore hole. Other proposals would rely on hot waste to melt the borehole and its contents which would then solidify. The borehole above the waste would be filled with clay, cement, crushed rock, asphalt and/or other materials to further isolate the waste.

          Nuclear transmutation: In this form of waste treatment, spent nuclear fuel is irradiated with fast neutrons in a reactor. Dangerous and long-lived transuranic radioisotopes such as plutonium, neptunium americium and curium fission and are transmuted into other elements which have shorter half-lives or are not radioactive.

          Other techniques of spent nuclear fuel disposal are being discussed and developed. However, none of the techniques covered above or other proposals has been selected as being the definitive safe economical method of permanent spent nuclear fuel disposal.

Underground storage diagram from geo.arizona.edu

Nuclear waste undeground.jpg

          Nuclear reactors burn nuclear fuel to generate electricity. Most reactors burn uranium oxide in the form of ceramic pellets in long tubes. The tubes comprise the core of the reactor where the fission reaction takes place. The zirconium cladding of the fuel rod tends to migrate into the center of the pellets while the lower boiling point fission products move to the edge of the pellet. Small bubbles form in the pellet which fill with cesium-137 from decaying xenon.

         Three percent of the mass of spent nuclear fuel consists of the fission products of U-235 and Pu-239. Fission products include every element in the periodic table from zinc up to the lanthanide series. Some of the fission products are either non-radioactive or have short half-lives. Other fission products are long-lived radioisotopes including strontium-90, cesium-137, technetium-99, and iodine-129.

         One percent of the mass of spent nuclear fuel is Pu-239 and Pu-240 which results from conversion of U-238. Pu-239 is a concern because it might be used in the creation of nuclear weapons. If the reactor is run for a long time, the amount of Pu-240 is greater than twenty percent and the proportion of Pu-239 is much less useful for weapons development. If the fuel is only irradiated for a short time, the proportion of Pu-239 is suitable for weapons production.

         Ninety six percent of the mass of spent nuclear fuel is uranium, with just under ninety five percent being the original U-338. About eight tenths of a percent is unburned U-235 and four tenths of a percent is U-236.

        Minor actinides including neptunium americium and curium will be present in minute amounts in spent nuclear fuel. The exact proportions will depend on the exact type of nuclear fuel being burned in the reactor. MOX or Uranium/Thorium fuels will produce slightly different rations of the actinides.

        When natural uranium nuclear fuel is burned in a reactor, it starts out with about seven tenths of a percent of U-235. When it is considered spent and removed from the reactor, it still have about one quarter of one percent of U-235 and one quarter of one percent of Pu-239. It is spent not because there is no fissile material left in the fuel but because the buildup of fission products has begun to absorb so many neutrons that the ability of the fuel to sustain a chain reaction has dropped to low to be useful.

        When spent fuel is removed from the reactor, it continues to generate fading amounts of heat due to beta emission of the fission products. Spent nuclear fuel that has been removed from the reactor core is stored in spent fuel pools of water. Most such pools circulate water around the fuel and then through a heat exchanger in order to dissipate the heat. The water in the pool also provides protection against the radiation emitted by the spent fuel. Current estimates are that the exiting spent fuel pools will be full within five years at current use of nuclear fuel.

Spent nuclear fuel pool – picture from radioactivechat.blogspot.com:

Spent fuel pool.PNG

         I have already touched on radioactive waste in several earlier posts but I wanted to treat the subject in a more systematic and thorough way. I am going to start with a breakdown of radioactive wastes into six categories.

         Spent nuclear waste from nuclear reactors: Most of the fuel for nuclear reactors is in the form of ceramic pellets of uranium oxide which are packed into long metal rods. A few thousand of these rods are joined together into what is called a “fuel assembly.” When the fuel in the assembly is exhausted and can no long power the reactor, the assembly is removed. Because there is no permanent storage or disposal solution for spent nuclear fuel, the spent fuel assemblies are usually stored in pools of water which dissipates heat from the assembly and blocks the radiation emitted by the rods.

          High-level waste from spent fuel reprocessing: Some spent fuel is dissolved in corrosive chemicals and then unfissioned uranium and plutonium are recovered from the resulting liquid. Ninety nine percent of the waste from reprocessing is generated by defense related nuclear weapons research and development. There are plans to solidify this waste but for the time being the waste is stored in liquid form in underground steel tanks in facilities in Idaho, Washington, South Carolina and New York.

         Transuranic Radioactive Waste: Transuranic elements are man-made elements that lie above uranium in the periodic table. Transuranic wastes are mainly generated by the production of nuclear weapons. Plutonium is the most prominent transuranic in such wastes. Also included in this category are contaminated clothes, tools, lab equipment, and containers.

          Uranium mill tailings: When uranium is mined and milled, huge piles of waste are generated which generally remain in the area of the mill. Radium is the most important radioactive material in the mill tailings. It decays into radioactive radon gas which diffuses into the atmosphere. Mill tailings also include selenium, molybdenum, uranium and thorium.

          Low-level radioactive waste: Low-level radioactive waste includes contaminated industrial or research waste. This includes clothing, packaging materials, organic fluids, and other waste products that do not fall into the other categories in this list. This waste is generated by DOE research facilities, private industry, nuclear plants, universities and other institutions.

         Naturally occurring and accelerator produced radioactive waste: There are naturally occurring radioactive materials that can be concentrated by human activity and pose a hazard. More diffuse naturally occurring wastes come from multiple sources including metal mining and processing waste, coal ash, phosphate waste, oil and gas production waste and water treatment residue and soil removed from above a deposit of uranium ore. Particle accelerators use magnetic fields to accelerate nuclear particles to high velocities for research, medical and industrial purposes. Short lived radioactive waste can be produced by such accelerators.

        In future posts, I will go into more detail on each of these types of waste including the problems and dangers they pose and methods of storage and disposal.

Ionizing radiation warning symbol created in 2007 by the International Atomic Energy Agency and the International Organization for Standardization:

Ionizing radiation warning symbol.png

          The Coalition Against Nukes (CAN) is a citizens group dedicated to ending nuclear power. Their mission statement includes the following ideas:

            They are working to phase out all nuclear weapons and nuclear reactors. They are concerned about the tons of nuclear waste being generated by reactors that cannot be safely stored and disposed of and will be dangerous to life for thousands of years. They believe that the fact that nuclear waste produced by reactors can be used to develop nuclear weapons is a threat to world peace and stability. They are promoting the development and use of alternate sources of energy such as wind, sun, geothermal sources and flowing water. Conservation and boosting efficiency are also on their agenda. The core tenant of the organization is that all nuclear issues from uranium mining and refining to power generation to weapons proliferation must be approached as a single issue. Their activities include networking with elected officials, creating a circulating petitions, organizing peaceful marches and rallies, and disseminating information about why the nuclear industry must be stopped. They have a Nonviolent Action Code of Conduct on the website.

         Their media outreach includes press releases on nuclear issues, press kits on nuclear issues, lists of speakers at their events, downloadable posters advertising their events, rally event guides with transportation advice. The website includes a membership section where you can sign up for a newsletter and event bulletins. They also have a listserve which people are invited to join.

            They encourage people to take direct actions by writing letters to congressmen. There is a sample letter that can be downloaded and fields that can be filled in with your specific information. They have a nuclear free community program that provides a downloadable resolution document of concern about the Indian Point nuclear power plant. They suggest using the resolution document, which has been signed by 80 communities, as a template for drafting your own resolution of concern about a particular nuclear power plant in your area. And they encourage people to volunteer for CAN.

            Their donation page has a section for direct donations of under $250 via check, credit card or PayPal and another section for indirect donations through another organization called the Nuclear Information and Resource Service.

            September 20,21 and 22, CAN held the Rally for A Nuclear-Free Future, a major event in Washington, D.C. that was an umbrella for a number of different events including a big public rally at the Capitol, a congressional briefing, an evening of speakers and live music at Busboys and Poets, a protest at the Nuclear Regulatory Commission national offices, a presentation of anti-nuclear films at the Letelier Theater, and a ceremony about nuclear impact on Native Americans at the Museum of the American Indian.

           Congressman Dennis Kucinich sponsored the Congressional Briefing for elected Representatives that was part of the CAN program to promote greater awareness about critical issues complicating the use of nuclear power for energy generation.

The CAN website is http://www.coalitionagainstnukes.org

The CAN logo:

CAN Logo.PNG

          There has been research in developing small thorium engines for consumer products like cars. Interest in such engines has increase recently with the concern about the decline in the oil supply and global warming. It is estimated that one gram of thorium contains the equivalent energy found in 7,500 gallons of gasoline.

          Cadillac debuted the Thorium Fueled Concept car designed by Loren Kulesus at the 2009 Chicago Auto Show. The design called for redundancy in all the major systems so that if a part failed, a backup part would take over. The car was supposed to last 100 years without need for replacement of parts or refueling. The design concept called for a small thorium powered nuclear reactor but the vehicle did not contain a real thorium engine. The reactor was to be contained in a study housing to prevent accidental or intentional tampering and had multiple systems to kill the reaction in case of any problems.

          A MaxFelaser is an example of a next generation table top accelerator that can support electrical fields much stronger than the old accelerators required. The thorium is inserted into a cavity laser. The alpha and beta particles released by the thorium triggers the lasing process. Power generation has been explored by the heating of a plasma and the utilization of magneto-hydrodynamics. Such generators have the potential to be small and powerful.

          It was announced last year that Laser Power Systems in Massachusetts is working on a prototype of a thorium powered “MaxFelaser” generator that could be used to generate enough energy to power a car which would produce no emissions. They says that the thorium will be heated to fire the MaxFelaser beam which will turn water into pressurized steam. The steam will turn a turbine that produces electricity. The steam will be condensed back into water and recycled in a closed system. A 250 kilowatt thorium power system will weigh around 500 pounds and be small enough to drop into the engine well of a regular sized car. The car is would be powered by feeding the generated electricity to motors in the hubs of the wheels.

          If such a power generation system can be developed it could be used for a myriad of other purposes such as powering buildings, ships, airplanes, trains, factories, etc. The problem with the Laser Power Systems claims is that there is very little information available about how their system actually works. Their website contains a lot of positive descriptions of the possibilities but very little technical detail. What I have read there does not make a lot of sense. I will feel much better about the reality of a thorium MaxFelaser generator when LPS actual puts a commercial unit on the market. Until then, I am afraid that I remain skeptical about the claims being made by Laser Power Systems.

Cadillac Thorium Fueled Concept car from autoblog.com:

thorium Cadillac.jpg